Underground devices and cables may be very sensible to underground lightning electromagnetic field. And some of them even connected to power electronic system. The underground electromagnetic field induced by lightning can be analytically expressed by Sommerfeld integrals, but it requires a lot of computational effort. In some published paper, approximate expressions have been proposed to quickly evaluate the electromagnetic field. But for the complex and stratified media, the time domain numerical approaches were more suitable. In this paper, the finite-difference time-domain (FDTD) method was employed to calculate the lightning electromagnetic field under the ground in cylindrical coordinates. The changing trend of electromagnetic field in different media under the ground has been found. We also analyzed the ups and downs of electromagnetic field's peak at the same distance from the strike channel. Finally, the distribution of peak value in whole region is obtained.
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